1. Field of the Invention
The present invention pertains to the encapsulation of integrated circuits.
2. Description of the Prior Art
Certain users of integrated circuits wish to be supplied with circuits in the form of continuous flexible strips (in rolls) bearing these integrated circuits.
This makes it easier for these users to automate the mounting of these integrated circuits on printed circuit boards or on other supports. This is especially the case with integrated circuits designed for surface-mounting. The surface-mounting of components is a type of mounting be automated far more easily than the mounting of components by insertion in holes drilled in printed boards.
Surface-mounting machines are fed with components in the form of coiled strips that carry components. These strips are carried forward step by step as with a cinematographic spool. It is only when the individual component is being mounted on a printed circuit that it is separated from the rest of the strip.
There are two main types of manufacturing processes for integrated circuit chips.
In the first type of manufacturing process, a cut-out metallic leadframe (a copper leadframe for example) is used as a supporting strip is used. For each component position, this leadframe has, firstly, a socket to receive an integrated circuit chip and, secondly, individual conductors, which act as external connection pins for the integrated circuit. To make the leadframe retain sufficient mechanical strength, the conductors forming the external pins are connected to one another by elements of the leadframe and they are therefore short-circuited with respect to one another. It is only when an individual component is being detached in order to be mounted that the pins are unshorted by sectioning the elements which connect the conductors to one another.
In the second type of manufacturing process, a dielectric film (generally a polyimide) is used as the supporting strip. On this support, a pattern of thin film conductors is formed. Here again, these conductors here again act as external pins for the component when it is being mounted on a printed circuit. The conductors are printed on the dielectrical film. Hence, it is the dielectrical film that gives them mechanical strength, even if, in certain places, this film has openings over which the conductors pass. It is not necessary to provide for joining elements between the various conductors to ensure their mechanical rigidity.
In practice, there are two major problems here. The first problem relates to the testing of the integrated circuits and the second relates to their protection.
As regards the test, with strips made from a cut-out metallic leadframe it is impossible, in principle, to test the circuits so long as they are on the strip since their pins continue to be short-circuited until they are separated into individual components. The test (on the wafer) can be done on the integrated circuit chip before this chip is fixed to the leadframe and connected to the conductors of this leadframe. The test could also be done after separating an individual component when it is mounted on a printed circuit, but that would be to late since it is preferable to supply the user with rolls of already-tested components.
Moreover, this is one of the reasons why there is a need felt for mounting the integrated circuits on dielectric films because, in this case, the pins are not short-circuited and the circuits can be tested on the strips.
With respect to the problem of protecting integrated circuits, these circuits are known to be chemically and mechanically very fragile and it is obviously out of the question that a user should be supplied with a strip of integrated circuits in which the circuit chips are not protected in one way or another against mechanical shocks, corrosive agents and, if necessary, against electromagnetic radiation.
For strips made from a conducting leadframe, this raises no problem: the protection is got by encapsulation in a thermosetting resin. The technique is the one currently used to make integrated circuits in plastic packages. This technique is the transfer molding technique in which the leadframe bearing the chips is placed in a mold and in which a thermosetting resin is injected around the entire piece.
Experience shows that this method of protection, as used conventionally to encapsulate integrated circuits in plastic packages, does not at all lend itself to the encapsulation of circuits mounted on dielectric strips. For the thermosetting resin is injected at a high temperature and irreversibly changes the shape of the dielectrical strip. The strip with its shape changed is not satisfactory in appearance and is unsuitable for winding in tight coils.
To shield integrated circuits mounted on dielectrical strips, a simpler method than that of transfer molding is used. A drop of a polymerizable resin is deposited on the chip. This drop of resin coats the chip and the ends of the printed conductors connected to the chip. However, the mechanical protection and protection against radiation obtained are far less than with a thermosetting resin. Sometimes even certain parts of the chip, for example the rear and the sides, remain bare. Furthermore, depositing a drop of resin does not at all make it possible to form a component with calibrated dimensions as would be the case with molded packages. The resin is shapeless and spreads in varying degrees, the total quantity deposited is variable, etc.
A complicated and expensive method has already been proposed in the prior art to reconcile the contradictory requirements of testing and protective quality, where the former requirement entails the choice of a dielectrical strip with printed conductors and the latter entails the choice of a cut-out metallic strip. In this prior art method a metallic leadframe is used, and the thermosetting resin molding operation entails making not only a plastic package that encloses the integrated circuit chip and lets the external connection pins, short-circuited by connecting bars, pass through, but also a peripheral plastic ring around the package. The external connection pins then extend between the package and the peripheral ring. They are rigidly held by the package on one side and by the ring on the other side, and they can then be unshorted by sectioning the connecting bars without affecting the mechanical strength of the strips. Outside the peripheral ring, testing pads connected to the connecting pins are held in place by the ring. The individual testing of each component is made easier by these pads. However, this test is possible only after separating the components into individual elements and cannot be done so long as they are in strips. The protection is of high quality since it is done by molding a setting resin. The disadvantage of this method is its high cost, since the molds to be used are complicated, and the bulkiness that results from the use of peripheral ring is quite great. Furthermore, tests cannot be conducted on strips.
The present invention is based on the experimental observation that any change in the shape of the dielectrical strip which makes the transfer molding of a thermosetting resin impossible in principle, can be prevented by using a different molding operation from the usual one.
The method still used in prior art techniques for the encapsulation of integrated circuits in thermosetting resin is a molding between two molding plates which can be separated and which are closed after the conducting leadframe bearing the chips is placed between these plates. The thermosetting resin is injected through the parting plane between the two molding plates. This makes it easy to clean the molding plates and the resin injection hole after the molding has been done and after the strip of encapsulated components has been removed from the mold.